The present invention relates generally to the removal of pollutants from a combustion gas stream, such as, for example, the combustion gas stream from a coal-burning power plant, from a sewage sludge combustion plant or from a garbage or waste material combustion plant, and more particularly to the removal of mercury from such a combustion gas stream.
The combustion gas stream from a coal-burning power plant is a major source of airborne mercury. Coal contains mercury sulfide which is decomposed to elemental mercury in the combustion boiler. Downstream of the boiler, in the ducts and smoke-stack of the combustion system, and then in the atmosphere, part of the elemental mercury is reoxidized, primarily to mercuric chloride (HgCl.sub.2), by reacting with chloride ions or the like normally present in combustion reaction gases flowing through the combustion system of a coal-burning power plant.
A good-sized power plant will each day emit about a pound of mercury, as elemental mercury and mercury compounds. The concentration of mercury in the stream of combustion gas is about 7,000 ng/m.sup.3.
Mercury compounds and elemental mercury escaping from the stack of a power plant are transported in the air, and eventually fall back to earth at various distances from the stack. When such a material reaches the aquatic system (streams, lakes and the like) some of it is methylated. The resulting methylated mercury compounds are readily incorporated into the aquatic food chain. There, the mercury level bioaccumulates in concentrations from one million to ten million times greater then the environmental concentration outside the aquatic food chain. Such concentrations of mercury are harmful to species in the aquatic food chain and also pose dangers to humans who consume the aquatic species.
Past efforts to remove mercury from the stream of combustion gas, before it leaves the stack of a power plant, include: (a) injection, into the combustion gas stream, of activated carbon particles or particulate sodium sulfide; and (b) flowing the combustion gas stream through a fixed bed of activated carbon particles.
When activated carbon particle injection is employed, the mercuric chloride in the gas stream is removed from the gas stream in a bag house and collected there as part of a powder containing other pollutants in particulate form. Mercuric chloride can be more readily removed from the gas stream at a bag house than can elemental mercury.
Sodium sulfide particle injection can be utilized to form mercuric sulfide (HgS) which is more readily removable from the gas stream at a bag house than is elemental mercury.
When the gas stream flows through a bed of activated carbon particles, mercury compounds are adsorbed on the surface of the activated carbon particles and remain there. Elemental mercury, usually present in vapor form in combustion gases, is not adsorbed on the activated carbon to any substantial extent without first being oxidized into a compound of mercury.
All of the expedients described in the preceding three paragraphs present solid waste disposal problems. The powders collected at the bag house, and the spent activated carbon removed from the bed of activated carbon, all contain mercury compounds and thus pose special problems with respect to burial at land fills where strictly localized containment of the mercury compounds is imperative. The concentration of mercury compounds in powder collected from a bag house is relatively minute; therefore, a very small quantity of mercury would be dispersed throughout relatively massive volumes of land fill, wherever the bag house powder is dumped. Moreover, with respect to activated carbon, that material is relatively expensive, and once spent activated carbon particles are removed from an adsorbent bed, they cannot be easily regenerated and used again.
Another expedient for removing mercury compounds from a stream of combustion gas involves the use of scrubbers. Mercury compounds are normally present in the combustion gas stream as vapors which can be removed from the gas stream by directing the stream through a scrubber where the mercury compounds are incorporated into a scrubbing solution. Spent scrubbing solution is withdrawn from the scrubber and has a relatively large liquid volume with only traces of mercury compounds therein; these compounds must be removed from the spent scrubbing solution before the waste liquid from this solution can be released into the environment. Removing traces of mercury compounds from large volumes of spent scrubbing solution is expensive and not commercially practical.
A drawback to all of the expedients described above is that they remove from the gas stream substantially only mercury compounds. A substantial part, if not most, of the elemental mercury in the gas stream remains unoxidized and, as such, is not removed by any of these expedients.
A further expedient employs a combination of sulfur and activated carbon particles; the sulfur chemically bonds with elemental mercury vapor forming mercuric sulfide which remains on the activated carbon particles. Sulfur impregnated alumina particles have also been employed; the sulfur reacts with elemental mercury vapor to form mercuric sulfide which remains on the alumina particles.